The present invention relates to the electrolytic production of metals, such as aluminum. More particularly, the invention relates to electrolysis in a cell having a cermet inert anode comprising a ceramic phase and a metal phase, wherein the anode has been subjected to a chemical milling process.
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,399,088; 4,455,211; 4,582,585; 4,584,172; 4,620,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799, and application Ser. No. 09/629,332 filed Aug. 1, 2000, assigned to Alcoa Inc. These patents and application are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000xc2x0 C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperatures, i.e., about 900 to 1,000xc2x0 C., so that the voltage drop at the anode is low.
Although the use of inert anodes in aluminum electrolytic reduction cells has been proposed in the past, the use of such inert anodes has not been put into commercial practice. One reason for this lack of implementation has been the long-standing inability to produce aluminum of commercial-grade purity with inert anodes. For example, impurity levels of Fe, Cu, and/or Ni have been found to be unacceptably high in aluminum produced with known inert anode materials. Thus, in addition to the above-noted criteria, aluminum produced with inert anodes should not be contaminated with constituents of the anode material to any appreciable extent. The present invention has been developed in view of the foregoing.
Cermet inert anodes generally contain metals on their outer surfaces. The metals can be, for example, part of the metal phase of the anode, or metals that have bled to the surface of the anode during the sintering step of their production. These metals dissolve in the Hall bath during electrolysis and end up as impurities in the aluminum or other metal being produced. The present invention provides a cermet inert anode that has been chemically milled to reduce, if not eliminate, the presence of contaminating metals on the surface of the anode. Reduction or elimination of these contaminating surface metals results in an aluminum product that is of a commercial purity.
It is, therefore, an aspect of the present invention to provide a method for making a cermet inert anode, in which said anode is subjected to a chemical milling procedure that reduces, if not eliminates, contaminating metals from the surface of the anode. A further aspect of the invention is to provide the cermet inert anodes that result from this method, which anodes are suitable for use in a molten salt bath.
A further aspect of the invention is to provide an electrolytic cell for producing metal. The cell includes a molten salt bath comprising an electrolyte and an oxide of a metal to be collected, a cathode, and the cermet inert anodes of the present invention.
Another aspect of the invention is to provide a method for producing commercial purity aluminum, utilizing the cermet inert anodes of the present invention.
Other aspects and advantages of the invention will be apparent to those skilled in the art based upon the following detailed description and appended claims.